In the clinical analysis of blood, one of the more important and frequently performed tests that are run is the one for blood urea nitrogen (BUN). BUN is tested to determine, e.g., abnormal kidney behavior. Conventionally, such a test relies upon the conversion of the urea into ammonia gas, and the reaction of a leuco dye and the gas to form a dye, the density of which is proportional to the amount of urea present.
A recent improvement in the BUN tests is the use of dry chemistry test elements to conduct the test. As used herein, "dry chemistry" refers to the absence of liquid reagents, characteristic of wet assays, in the element or analyzer used with the elements, prior to use. Instead, all the reagents appear in a test element in one or more reagent layers, which are dry to the touch. A representative patent describing preferred dry chemistry BUN test elements is U.S. Pat. No. Re. 30,267 by Bruschi.
Although BUN test elements constructed in the manner described by the aforesaid patent have enjoyed great commercial acceptance, there remained prior to this invention a minor problem called "ammonia carryover." There are other dry chemistry test elements that measure ammonia gas, but in much smaller quantities. For example, a creatinine test element (also used to test for kidney malfunctions) uses creatinine iminohydrolase to convert the creatinine into ammonia gas, which is measured using a leuco dye having a higher coefficient of extinction than the one used in the Bun test element. Because it is inevitable that some ammonia gas will escape from the aforesaid BUN test element during use, if a creatinine test element is close by such a BUN test element during use, ammonia "carries over" into the creatinine element. There is a further aspect of the creatinine assay which exacerbates the problem. An ammonia blank measurement is made on the serum to subtract out the serum ammonia present, which is otherwise an interferant. The ammonia blank element also measures ammonia, as is readily apparent. Since both the creatinine element and the ammonia element blank are designed to be 10,000 times as sensitive to such gas as is the BUN element, any such carry-over presents a serious problem. The problem is particularly acute in analyzers, such as those described in U.S. Pat. No. 4,303,611, wherein the dry chemistry test elements are incubated in a stack each one next to and in contact with another.
This problem was known prior to this invention, but what was not known was its exact cause. Many attempts have been made to deal with this ammonia carry-over problem. For example, one possibility is to instruct the user to avoid testing the creatinine element in close proximity with a BUN element. Or alternatively, evaporation caps can be provided to attempt to keep ammonia from escaping from BUN elements. Also, vent holes were formed in the supports of the test elements, and non-absorbing surfaces have been provided in the analyzer.
None of those attempts have been very successful, particularly when the test elements are stacked as noted above. For example, venting the incubator support to allow released gas to escape to the atmosphere leaves unsolved the problem that the released gas will not be available to the indicator, thereby producing a reduction in sensitivity. Furthermore, warning the user to be careful about "close proximity" is not satisfactory, compared to finding a safeguard that can be built into either the test chemistry or the analyzer. No such satisfactory built-in safeguard had been found prior to this invention, particularly for analyzers that stack incubating elements. Even non-stacking analyzers would benefit from sensitivity improvement.